DRILL-3037: Fix impersonation issue with HDFS based filesystem plugins.
We have common code where we impersonate the owner of scan (proxy user) while creating
a filesystem object. When impersonation is enabled the proxy user is either query
user or view owner (if the query involves views). When impersonation is disabled the
proxy user is always the user who started Drillbit (processUser). This causes problems
as "processUser" tries to impersonate "processUser" when impersonation is disabled.
HDFS based filesystems which have impersonation disabled prohibit this (even the self
impersonation).
Fix is don't impersonate "processUser". If the given proxy user name is same as
"processUser", then return the process UserGroupInformation directly instead of
proxy UserGroupInformation.
